fix(state): scope all per-account localStorage by account id

Multi-account-in-one-browser (esp. with per-tab accounts) leaked one account's client state
into another via shared localStorage keys. Scope every account-specific key by the tab's active
account (accountKey/readAccount/writeAccount/useAccountPersistedState helpers), so nothing bleeds
across accounts or tabs:

- Real leaks: selected playlist, client notification history + settings, onboarding-dismissed.
- UI position: feed page, channel-manager filter/view + tables, playlist sort, Settings/Stats/
  Users/Config tabs.
- Previously DB-adopted caches (theme, hints, performance mode, sidebar layout, nav/filter
  collapse) — now the cache is per-account too, so there's no flash of the other account's value
  on login.

Kept intentionally global: siftlode.lang (needed pre-login on the Welcome page; the DB pref still
scopes it per-account after sign-in) and siftlode.seenVersion (a per-browser 'new version' banner).
E2EE private keys (IndexedDB) and the chat-dock key were already per-user.

frontend/src/lib/api.ts

@@ -1,6 +1,7 @@
 import { notify, remove } from "./notifications";
 import i18n from "../i18n";
 import { reportError } from "./errorDialog";
+import { activeAccountId, ACTIVE_ACCOUNT_KEY } from "./storage";
 
 export interface Me {
   id: number;
@@ -270,19 +271,10 @@ const setupHeaders = (token: string) => ({
 // The signed session cookie is the browser's account "wallet"; which account a given TAB acts
 // as is chosen here and sent per-request, so two tabs can run two accounts at once. Stored in
 // sessionStorage (per-tab, survives F5, not shared across tabs). null = use the session default.
+// The storage key + reader live in storage.ts (so its per-account helpers can use them too).
 const ACTIVE_ACCOUNT_HEADER = "X-Siftlode-Account";
-const ACTIVE_ACCOUNT_KEY = "siftlode.activeAccount";
 
-export function getActiveAccount(): number | null {
-  try {
-    const raw = sessionStorage.getItem(ACTIVE_ACCOUNT_KEY);
-    if (!raw) return null;
-    const n = Number(raw);
-    return Number.isFinite(n) ? n : null;
-  } catch {
-    return null;
-  }
-}
+export const getActiveAccount = activeAccountId;
 export function setActiveAccount(id: number): void {
   try {
     sessionStorage.setItem(ACTIVE_ACCOUNT_KEY, String(id));